In his inauguration speech on January 20. President Donald Trump promised “to launch the largest deportation program of criminals in the history of America.” The president also stated they he would “send troops to the southern border to repel the disastrous invasion of our country. . . [because] as commander in chief I have no higher responsibility than to defend our country from threats and invasions.”
The president knows that absent a massive new authorization and infusion of funds to ICE and CBP, the immigration infrastructure housed inside the Department of Homeland Security (DHS) cannot carry out these plans. Thus, the president’s determination to rely on military support for immigration policy could help make up the difference in financial and human resources needed to carry out the mission. Later on January 20, Trump signed ten executive orders on immigration and border enforcement that repeated some of the rhetoric but also identified legal authorities that would purport to authorize or fund a significant domestic deployment of U.S. military for immigration purposes.
One executive order, Declaring A National Emergency at the Southern Border of the United States, allows the Defense Department to deploy troops to secure the border. President George W. Bush and Barack Obama did much the same thing. More important, the national emergency declaration unlocks funds for further construction of Trump’s border wall, repeating a step he took in 2019. More ominously, the order requires a report from the Secretaries of Defense and Homeland Security within 90 days “regarding additional actions that may be necessary to obtain complete operational control of the southern border, including whether to invoke the Insurrection Act of 1807.”
A second executive order, Protecting the American People Against Invasion, revokes several Biden administration immigration enforcement policies; instructs DHS, its immigration agencies, and the Justice Department to hasten the removal of undocumented persons and the civil and criminal prosecution of immigration law violators; and orders DHS to “construct, operate, control, or use” sufficient removal detention facilities to implement administration immigration policies.
The invasion imagery is, of course, arresting. It conjures Americans’ worst fears, although for most of us the dread springs from apocalyptic Hollywood movies or dramatic tales from science fiction novels. To be sure, the “invasion” label was chosen and repeated for a specific reason having to do with the president’s legal authorities. Regardless of how one characterizes recent trends in migration at our southern border, describing the migration as an “invasion” does not assure the lawfulness of Trump’s determination to utilize the military in immigration enforcement.
The Alien Enemies Act and Potential Role for the Military
The Alien Enemies Act was one of four components of the Alien and Sedition Acts passed by Congress in 1798. The United States was on the brink of war with France and the dominant Federalist Party feared that non-citizens living in the United States would sympathize with France and threaten domestic security during a war. The president can invoke the Alien Enemies Act if Congress has declared war or if the president proclaims an “invasion or predatory incursion … by a foreign nation or government.” Once invoked, the Act authorizes the president to detain and deport any such non-U.S. citizens who were born in the enemy nation, including those lawfully present in the United States, regardless of any reason to suspect that they are dangerous.
The Alien Enemies Act was last used during World War II to intern tens of thousands of non-U.S. citizens of Japanese, German, and Italian descent at military facilities. (The infamous military detention and internment of Japanese-Americans was separately authorized and then upheld by the Supreme Court.) Decades later apologies were issued and reparations paid by Congress for their role in this shameful chapter of wholesale discrimination against people based on their ancestry.
Now, the strongest argument that the Trump administration might attempt is that migration from Mexico or other Latin American countries amounts to an “invasion” or “predatory incursion” at the behest of drug cartels that operate as de facto governments in those regions. On that theory, the Alien Enemies Act permits him to deport Latino immigrants who are undocumented or involved in criminal drug activity indeed, even if they are legally present.
Although the Alien Enemies Act does not authorize military participation in law enforcement, if other laws (reviewed below) permit their use, Trump could enable the military to implement summary detention and deportation of immigrants lawfully in the United States.
When May the Alien Enemies Act be Invoked?
The historical context and structure of the Alien Enemies Act make clear that its terms apply in wartime only. The 1798 Congress sought to address feared armed attacks by a foreign power (France), not individuals fleeing persecution or pursuing economic opportunity. Contemporaneous history and usage of the terms “invasion” and “predatory incursion” confirm the original understanding. James Madison wrote in 1800 that “invasion is an operation of war,” and that while a “predatory incursion” was understood as more circumscribed than an invasion such smaller attacks were known to cause “great destruction.”
Historical examples of the uses and abuses of the Alien Enemies Act confirm that it applies only in wartime and when the invasion or incursion occurs at the behest of a “foreign nation or government.” Drug cartels are hardly de facto governments or foreign nations. The infamous reliance on the Act to intern innocent migrants based on nothing more than their ancestry during World War II was until now the last time the Alien Enemies Act was invoked.
There is no doubt that the Alien Enemies Act confers broad powers on the president. Such expansive authorities are not uncommon for wartime exigencies. However, a reading of the Act to support migrant deportation is at odds with centuries of legislative, presidential, and judicial practice, all of which confirm that the Alien Enemies Act is a wartime authority.
In addition, the few instances where the term “invasion” is found in the Constitution confirm its common usage to refer to intrusions by use of military force. The Constitution empowered Congress to “call[] forth the Militia to … repel Invasions,” but text and historical context underscore that the term anticipated possible hostile incursions by a foreign power, not border crossings by migrants. Similarly, Article IV, Section 4 of the Constitution requires that the “United States shall … protect each [State] against Invasion.” The so-called “Guarantee Clause” was widely understood at the Framing and during ratification to be a bulwark against “foreign hostility” and “bloody” events involving “military talents and experience” and “an appeal to the sword.” Similarly, Article 1, Section 10 prohibits states from engaging in war, “unless actually invaded, or in such imminent danger as will not admit of delay.”
The Importance of Framing Migration as an Invasion
Sometimes rhetoric speaks more forcefully than law. The Trump administration efforts to tar migrants as invaders surely fails to unleash the extraordinarily broad authorities to detain migrants pursuant to the Alien Enemies Act, much less a proper justification to deploy the military to conduct immigration enforcement. Yet the impetus for mass deportations remains, with apparent wide public support, even if in search of legal authority. Framing recent migration as an “invasion” might thus still advance the president’s cause as administration officials begin to implement their enforcement and deportation plans. What follows is a brief sketch of some of the pertinent authorities that prescribe the military role in law enforcement, the pitfalls in their use, and their likelihood of achieving the President’s objectives.
Background Principles
The United States has historically been widely respected for our tradition of entrusting law enforcement to civilians – our federal agents, local and state police, sheriffs, constables. By contrast, our uniformed military fight wars and keep us safe from foreign adversaries and are only episodically needed for domestic assignment. Our traditions are reflected in the Constitution, as noted above, and are explained in part by antipathy to the English Crown and the heavy-handed use of the British military in the colonies. While the Constitution enabled Congress to Call Forth the Militia (today’s National Guard) to “repel Invasions,” the grant of authority was understood to anticipate the possibility of war being brought to the United States.
In 1878 Congress enacted the Posse Comitatus Act (PCA). The PCA created a statutory presumption against military participation in law enforcement, a presumption that could only be overcome by other legal authorities found in statute or the Constitution. Despite the lack of criminal prosecutions for violation of its terms, the PCA has led to a longstanding and pervasive orientation in the U.S. military to steer clear of civilian law enforcement except pursuant to express statutory authorization or to repel a sudden attack on the United States.
A few of those statutory exceptions to the presumptive ban in the PCA loom large in the debates over the military’s involvement in immigration enforcement. It’s also important to note that the PCA applies only to federal armed forces, not to National Guard units unless federalized. State governors control their National Guard units when deployed in their default state status. In addition, the PCA prohibits direct participation in core law enforcement, but indirect support, such as intelligence sharing and providing equipment is allowed (measures that are included in the Emergency Declaration Executive Order). The PCA also does not apply to activities by the military that have a primarily military purpose.
Indeed, if the “invasion” rubric had more than rhetorical force under the Alien Enemies Act, the PCA might not apply. That’s because the executive action could then theoretically become a military operation, not one of law enforcement. It is important to pay attention to one of the executive orders – Clarifying the Military’s Role in Protecting the Territorial Integrity of the United States – which, unlike the others, may be laying the groundwork for that approach.
Military Support for Civilian Law Enforcement Agencies
Several provisions of Title 10 of the U.S. Code authorize federal military assistance to law enforcement agencies, including immigration enforcement. The assistance may include the provision of equipment, personnel for training, expert advice, and military support for counterdrug activities, including military support at the southern border. These authorities have been used across several administrations. Notably none of them authorize direct participation in core law enforcement activities, presumably including search, seizure, arrest, and detention.
In his first term President Trump relied on these authorities to support his actions at the southern border. Because there was no appropriation for these border activities, Trump ordered the Defense Department to utilize its discretion to reprogram funds allocated by Congress for other programs. Although a second Trump administration is again trumpeting the invasion rhetoric to support DoD reprogramming, there are statutory limits that would have to be lifted by Congress to permit significant new spending.
The National Emergencies Act of 1976
The National Emergencies Act (NEA) authorizes the president to declare a national emergency which, once declared, triggers various emergency authorities prescribed in about 150 laws. Many of the statutes enable enhanced authorities for immigration enforcement. In recent years Presidents Trump and Biden relied on the NEA to invoke authorities that permitted call up of armed forces reserve components, including the National Guard, to combat drug trafficking and, in Trump’s first administration to close most cross-border traffic at the southern border. Trump followed suit with a NEA declaration on the first day of his second term. Most notably for Trump’s second term initiatives, one of the emergency authorities allows the Secretary of Defense to undertake military construction projects in a national emergency “that requires the use of armed forces.” Funds may be transferred from military projects that have been canceled or downgraded toward funding, for example, the construction or modification of military bases to serve as immigrant detention facilities. Like the civilian support authorities mentioned above, none of the emergency powers authorize direct participation in core law enforcement activities. In any case, the panoply of emergency powers may well amplify the resources available to the President for immigration enforcement. Moreover, because the NEA does not define “emergency,” any judicial challenge to President Trump’s invocation of the NEA will likely defer to the president’s judgment that an emergency exists. Courts could in any case consider whether the proposed executive actions are authorized by the specific emergency authority invoked. (For more on this topic, I recommend Robert Taylor, A Caution to Pentagon Officials Asked to Obligate “National Emergency” Funds.)
The National Guard and Title 32
In their Title 32 status, state National Guard forces may perform a federal mission – such as immigration enforcement – but remain subject to the governor’s command. Importantly, Guard forces so deployed are not subject to the PCA, such that the soldiers may carry out core law enforcement tasks. During the administrations of Presidents George W. Bush, Obama, Trump, and Biden, Title 32 National Guard forces were deployed at the border. None of their operations included core law enforcement activities but instead involved surveillance, transportation, provision of equipment, and the erection of barriers. That could change in the second Trump administration, but the initial flurry of executive orders authorized only indirect military support to law enforcement.
That said, there are constraints on the uses of the Guard under Title 32. State governors may lawfully refuse the president’s request to deploy troops for a federal mission (more below on federalizing the Guard not upon request, but command). Also, a governor may not send Guard forces into another state without its governor’s consent. In essence, then, Title 32 can support Guard operations only within states that have governors who support Trump’s policies.
The Insurrection Act
The Insurrection Act allows the President to deploy active-duty military and federalized National Guard to enforce federal law or suppress a rebellion against federal authority whenever the president determines that “unlawful obstructions, combinations, or assemblages, or rebellion against the authority of the United States” make it “impracticable” to enforce federal law “by the ordinary course of judicial proceedings.” A lengthy Policy Brief prepared by former and likely future Trump administration lawyers parrots the language of the Act and argues that a migrant “invasion” satisfies the Insurrection Act predicate, making it “impracticable to enforce the laws of the United States.” The Insurrection Act thus allows the President to use the National Guard or units of the regular military following his determination of the impracticability of enforcing the laws with civilian resources. No state officials may stand in their way because the Guard units are operating as federal armed forces. The threshold issue here is whether the claimed predicate of a migrant “invasion” has sufficient legitimacy to it, and as explained above, it does not.
The Insurrection Act has not in the past been relied on to authorize immigration enforcement or border security. However, Trump’s executive order Declaring a National Emergency at the Southern Border of the United States specifically calls for the Secretaries of Defense and Homeland Security to report to the president “regarding additional actions that may be necessary to obtain complete operational control of the southern border, including whether to invoke the Insurrection Act of 1807.” While I have argued in a 2009 law review article that using federal or federalized armed forces to quell domestic violence without a state’s request for federal assistance violates the Guarantee Clause of Article IV of the Constitution, that argument is unlikely to hold sway in the context of the Trump immigration enforcement agenda.
Conclusion
Throughout his first term and his 2024 campaign, Trump himself or through his surrogates painted undocumented migrants as part of an invasion, destined to “poison our blood” as a nation. As rhetoric, “invasion” conjures images of violence and mayhem and strikes fear and dread among many perhaps more impressionable citizens. As underscored in his inaugural address and executive orders issued on January 20, Trump stoked those same flames. Now that he is president again, he has considerable though not unlimited authority to follow through on his promises to seal the southern border, reduce the influx of migrants, and remove undocumented immigrants from within the country. To the extent he relies on the U.S. military to help carry out his more expansive aims, the president will be testing the longstanding orientation of our military to be uninvolved in domestic law enforcement. There are legal guardrails that have long served to prevent civilians from misusing military power. Let’s hope that those guardrails remain firmly in place.
Editor’s note: This piece is part of the Collection: Just Security’s Coverage of the Trump Administration’s Executive Actions